In recent years, an image forming apparatus using an electrophotographic method is required to meet the needs for higher speed and higher image quality. To meet the requirement for higher speed, the linear speed of an image carrier such as a photoreceptor must be increased. If the linear speed of the image carrier is increased, however, various forms of problems will arise in the process of development.
When the linear speed of the image carrier is increased, the major problem related to image finality in the process of development is that the width of the settable fogging margin known as the “operation window” is narrowed.
The fogging margin is the potential difference between the charging potential of the image carrier (potential of the unexposed portion in the photoreceptor) and the DC component of the development bias potential. The fogging margin is generally set to a value wherein neither fogging nor carrier deposition should occur.
As the fogging margin is greater, generation of fogging is reduced, but deposition of carrier is more heavily caused by the counter-charge phenomenon that occurs in the development area at the time of development. The counter-charge can be defined here as a phenomenon in which carrier is charged by exchange of electric charge between the toner having been developed (transferred) from the development roller to the carrier and the carrier carried by the development roller.
Development is performed by two methods: a with-rotation method where development is performed by moving the image carrier and development roller in the same direction in the development area where they are opposed to each other; and a counter-rotation method where development is performed by moving the image carrier and development roller in different directions. From the viewpoint of reducing the carrier deposition, the counter-rotation method is superior. This is because in the development by the counter-rotation method, the moving direction of the electrically charged carrier is directed upstream of the development area, and a fresh carrier, which is not electronically charged, is supplied on the downstream side of the development area.
In the counter-rotation method, however, the relative speed between the development roller and image carrier is generally high. When a half-tone or solid black image is developed, toner will be swept to the edge of the image by the sliding of the magnetic brush formed on the development roller surface, and the density of that portion is higher than other portions. This is often called “gathered toner”, which reduces the image quality.
On the other hand, there is another development method which employs one-component developer, in which method toner charged without carrier is carried on a development roller to perform development. The development method using the one-component developer is superior in uniformity of image, but is inferior with respect to increasing speed and gradation reproduction.
In an effort to solve this problem, Patent Document 1 proposes an image forming apparatus using a development roller incorporating a magnet and carrying two-component developer, and a development roller carrying one-component developer. This image forming apparatus performs multi-stage development using two development rollers. This is intended to improve the development quality and to prevent a white patch by using two development rollers, and intended to prevent a phenomenon which disturbs images, by performing scavenging development, that is to say, mechanically or electrostatically scraping with carrier an image being carried.
Further, in the developing device disclosed in the Patent Document 2 that uses only the two-component developer for cost reduction purposes, where toner is supplied from a magnetic roller carrying the two-component developer to a one-component toner roller. This structure eliminates the need for a development chamber incorporating one-component developer.